It is desirable, for vehicle strengthening members, to maximize impact energy absorption and bending resistance while minimizing mass per unit length of the strengthening member.
When a compressive force is exerted longitudinally on a strengthening member, for example a force due to a front impact load on a vehicle's front rail or other strengthening member in the engine compartment, the strengthening member can crush in a longitudinal direction to absorb the energy of the collision. In addition, when a bending force is exerted on a strengthening member, for example a force due to a side impact load on a vehicle's front side sill, B-pillar or other strengthening member, the strengthening member can bend to absorb the energy of the collision.
U.S. Pat. No. 6,752,451 discloses a strengthening member having concave portions at the four corners of a basic rectangular cross section, resulting in four U-shaped portions forming an angle of 90 degrees with each other. To avoid cracks at the concave portions at the four corners and to increase strength, the concave portions have increased thickness and hardness. Increased thickness and hardness of the corner portions is disclosed to be achievable only by drawing or hydroforming, and therefore decreases manufacturing feasibility while increasing the mass per unit length of the strengthening member.
U.S. Pat. No. 6,752,451 makes reference to Japanese Unexamined Patent Publication No. H8-337183, which also discloses a strengthening member having concave portions at the four corners of a basic rectangular cross section, resulting in four U-shaped portions forming an angle of 90 degrees with each other. U.S. Pat. No. 6,752,451 states that its thickened concave portions provide improved crush resistance and flexural strength over H8-337183.
It may be desirable to provide a strengthening member configured to achieve the same or similar strength increase as provided by the thickened corners, while minimizing mass per unit length of the member and maintaining a high manufacturing feasibility.
It may further be desirable to provide a strengthening member that can achieve increased energy absorption and a more stable axial collapse when forces such as front and side impact forces are exerted on the strengthening member. Additionally, it may be desirable to provide a strengthening member that possesses improved noise-vibration-harshness performance due to work hardening on its corners.
Under certain conditions, axial collapse of a strengthening member can proceed in an unstable buckling mode that is initiated in a middle of the strengthening member before moving to a top of the strengthening member in a non-progressive manner. During quasi-static crash testing, an unstable collapse mode can occur because deformation occurs so slowly that it leaves enough time for the test to react to imperfections in the strengthening member. An unstable collapse mode increases the variation in crash behaviors among replicate samples and makes crash performance more difficult to predict.
It is desirable for longitudinal rails to collapse axially and progressively in a predetermined mode during a crash such as a rear or frontal impact. While a stable axially progressive folding mode is generally considered to be the most effective mechanism for energy absorption, it can be difficult to achieve in structures having thin-walled sections.